The present invention relates to a combined pneumatic-hydraulic press head with high actuation speed.
Mechanically actuated presses, hydraulically actuated presses and presses with combined pneumatic-hydraulic actuation are known. The latter type of press is becoming increasingly widespread on the market, since its particular actuation method, by requiring only the use of compressed air, allows significant advantages with respect to presses with exclusively hydraulic actuation.
Presses with combined pneumatic-hydraulic actuation generally comprise a head which is constituted by a substantially cylindrical body in which a main cylindrical chamber is defined; a main piston is axially slidingly accommodated in said chamber and its stem protrudes from one side of the head. This stem constitutes the element of the press which, associated with various types of tools, performs the various treatments for which presses are normally used, such as for example clamping, blanking, marking, straightening, riveting, calking, coining, bending, drawing, keying, etc.
The main chamber can be selectively connected, on opposite sides with respect to the main piston, to a source of compressed air or to the atmosphere so as to rapidly move, with a reduced force, the stem toward or away from the working surface on which the part to be treated is arranged.
In order to actuate the main piston with an adequate force during the actual working step, the main piston generally has a wing which extends on the opposite side with respect to the stem and can slide in a liquid-filled chamber.
Said liquid-containing chamber has a narrower portion in which the stem of another pneumatically-actuated piston can slide sealingly. The assembly constituted by the liquid-containing chamber, by the wing of the main piston and by the stem of said pneumatically-actuated piston constitutes a hydraulic press in which the actuation force of the stem which slides in the narrower portion of the liquid-containing chamber is multiplied and transmitted to the main piston in order to obtain an adequate force during the working step.
These types of presses with combined pneumatic-hydraulic actuation, despite having undeniable advantages with respect to presses with exclusively hydraulic actuation, have some problems.
Since the main piston is conceived like the piston of a double-action pneumatic cylinder, during the rapid approach step said main piston in fact draws the liquid contained in the chamber in which the wing which constitutes an element of the hydraulic press slides. This drawing action, allowed by a compensation element which sends liquid into said chamber or keeps its volume constant by means of another sliding piston which delimits the chamber on the side opposite to said wing, exerts a braking effect on the main piston, reducing its speed of approach to the part to be treated or requiring greater pneumatic power in order to obtain the required speed. Furthermore, the filling of the chamber of the liquid by drawing is never assured and complete in short time periods.
In some types of presses, in which the compensation element is constituted by a tank of liquid connected to the hydraulic press chamber, pneumatic pressurization of the tank reduces this effect but unavoidably makes the construction of the press more complicated and entails a reduction in the pressing hydraulic area.